Light transmissive metal oxide coatings may be applied to glass substrates by a number of dip-coating processes such as the well-known sol-gel process, wherein the glass substrate is immersed into a solution containing hydrolyzable metal compounds, the glass substrate is withdrawn at a specific, uniform rate into an atmosphere of known water content to form a gel on the surface of the glass substrate, and finally the substrate is heated to about 450.degree. C. to densify the gel to form a metal oxide coating. The thickness of the ultimately produced metal oxide coating is primarily determined by the concentration and viscosity of the solution and the velocity at which the glass substrate is withdrawn. Where the substrate is withdrawn slowly, a thin coating of gel adheres to the glass; where the glass substrate is quickly withdrawn from the coating solution, a thicker gel forms. This phenomenon of coating thickness versus withdrawal rate likewise is observed for coating processes other than sol-gel coating.
It is also well-known to vertically withdraw a substrate from a coating solution at a varying rate, so as to produce a coating which varies in thickness across the surface of the substrate in a direction parallel to the direction of withdrawal. Such a method has been used to prepare architectural glazings having a thicker glare-reducing coating at the top and a thinner glare-reducing coating near the bottom for unimpeded horizontal viewing therethrough.
U.S. Pat. No. 4,597,931 to Watanabe et al. discloses a method of manufacturing a windshield having a hard coat of varying thickness. The windshield is vertically withdrawn from a coating solution at a varying rate, so as to produce a thicker hard coat at an elevation where the windshield is contacted by a wiper blade, and a thinner hard coat at an elevation where the windshield attaches to the frame of the vehicle. The thickness of the coating varies in a direction parallel to the direction of withdrawal from the coating solution.
U.S. Pat. No. 4,455,322 to Weber discloses a method for coating lenses which likewise utilizes a variable speed for vertically withdrawing lenses to be coated from a coating solution, to prepare a coating which varies in thickness in a direction parallel to the direction of withdrawal from the coating solution.
It is well-known that anti-reflection coatings may be applied to optical or vision articles utilizing a sol-gel process. U.S. Pat. No. 4,271,210 to Yoldas discloses a sol-gel process, wherein a vitreous substrate is withdrawn at a uniform rate from an aluminum alkoxide solution and thereafter heated to form a uniform aluminum oxide anti-reflection layer thereon.
Anti-reflection coatings are particularly useful for the preparation of vehicle vision glazings; particularly windshields, for which maximum light transmission is desirable. Light reflected from a windshield is effectively not transmitted therethrough, and is therefore a transmission loss. As the angle of incidence of a windshield varies, when viewed from the eyes of a vehicle operator, the anti-reflection nature of the windshield (as well as the thickness of an anti-reflection coating) should also vary to provide optimal performance. Where a vehicle operator looks through the lower portion of a windshield, at a high angle of incidence, the anti-reflection coating should be thicker relative to the thickness of the anti-reflection layer at the top of the windshield where the operator looks through the windshield at a lower angle of incidence.
The application of an anti-reflection coating, having a differential thickness from the top to the bottom of a windshield, is very difficult because the glass blank to which such a coating is applied is in the form generally of a sector of an annulus. Therefore, the desired coating must have a constant thickness along any given arc of the annulus sector, but must vary from a thicker coating at the longest arc to a thinner coating at the shortest arc. Such a coating, which varies in thickness in more than one direction along the surface of the coated substrate, could not be applied by the well-known uniform vertical withdrawal or variable vertical withdrawal dip-coating techniques.
It would be desirable to devise a process for preparing a coating on a substrate by a dipping process, which coating has a differential thickness in more than one direction along the surface of the substrate. Such a process would be useful for differentially coating a vehicle windshield, to produce a windshield having a uniform anti-reflection coating thickness along any particular arc and a varying thickness from top to bottom.